Possible Quaternary Growth of a Hidden Anticline at The
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Possible Quaternary growth of a hidden anticline at the front of the Jura fold-and-thrust belt: geomorphological constraints from the Forêt de Chaux area, France Stéphane Molliex, Olivier Fabbri, Vincent Bichet, Herfried Madritsch To cite this version: Stéphane Molliex, Olivier Fabbri, Vincent Bichet, Herfried Madritsch. Possible Quaternary growth of a hidden anticline at the front of the Jura fold-and-thrust belt: geomorphological constraints from the Forêt de Chaux area, France. Bulletin de la Société Géologique de France, Société géologique de France, 2011, 182 (4), pp.337-346. 10.2113/gssgfbull.182.4.337. insu-00633369 HAL Id: insu-00633369 https://hal-insu.archives-ouvertes.fr/insu-00633369 Submitted on 24 Nov 2011 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Possible Quaternary growth of a hidden anticline at the front of the Jura fold-and-thrust belt: Geomorphological constraints from the Forêt de Chaux area, France. Stéphane Molliex (1, 2), Olivier Fabbri (3), Vincent Bichet (3), Herfried Madritsch (4, 5) (1) CEREGE, UMR 6635, Aix-Marseille Université, BP80, Europôle de l'Arbois, F-13545 Aix-en-Provence cedex, France (2) Now at: UMR 6538 Domaines océaniques, Institut Universitaire Européen de la Mer, Place Nicolas Copernic F-29280 Plouzané, France. E-mail: [email protected] (3) UMR 6249 Chrono-environnement, Université de Franche-Comté, 16 route de Gray, F-25030 Besançon cedex, France (4) Geological-Paleontological Institute Universtät Basel, Bernoullistr. 32, 4056 Basel, Switzerland. (5) Now at: NAGRA, Hardstrasse 73, 5430 Wettingen, Switzerland Key words: Jura fold-and-thrust belt, active tectonics, active folding, seismic hazard Published in 2011 in Bulletin de la Société Géologique de France, 182, p. 337-346, doi:10.2113/gssgfbull.182.4.337 Abstract This study presents new constraints for Plio-Quaternary (post-2.4 Ma to present-day) anticline growth along the frontal zone of the Jura fold-and-thrust belt, in the Forêt de Chaux area, located 30 km SW of Besançon. The Forêt de Chaux area consists of a N80°E-elongated depression bordered by the Doubs and Loue rivers to the north and south respectively, and filled with Sundgau-type Pliocene alluvial deposits. The upper surface of the Pliocene deposits between the Loue and Doubs rivers is marked by a N65°E-trending ridge crossing the depression in a median position. A differential uplift along this ridge, post-dating the deposition of the gravels (2.4 Ma), is suggested by several geomorphological observations such as the opposite river migration on each side of the ridge as well as variations of drainage geometry and incision intensity. Geological and geophysical subsurface data indicate that the ridge roughly coincides with the axis of an anticline hidden beneath the Pliocene deposits. The observed uplift is presumably related to a post-2.4 Ma anticline growth. The fact that the azimuth of the hidden anticline axis is parallel to the strike of deep-seated Late Paleozoic basement faults and not to the local strike of the thin-skinned Jura structures indicates that the inferred post-Pliocene deformation could possibly be an expression of a recent thick-skinned deformation of the basement of the northern Alpine foreland. The focal depth (15 km) of the February 24th, 2004, Besançon earthquake supports the hypothesis of a basement fault reactivation. Propagation possible d'un anticlinal masqué au front de la chaîne plissée du Jura au Quaternaire : indices géomorphologiques dans le secteur de la forêt de Chaux, France. Mots clés: chaîne plissée, Jura, tectonique active, plissement actif, risque sismique Résumé Cette étude présente de nouveaux indices en faveur d'une croissance, au cours du Plio-Quaternaire (post-2.4 Ma), d'un anticlinal situé au front de la chaîne du Jura, dans le secteur de la Forêt de Chaux à 30 km au sud-ouest de Besançon. La Forêt de Chaux correspond à une dépression allongée orientée N80°E et comblée par des dépôts alluviaux pliocènes de type Sundgau. Cette dépression est bordée au nord par le Doubs et au sud par la Loue. Le secteur situé entre ces deux rivières est marqué par la présence d'une ride orientée N65°E, qui traverse la forêt de Chaux d'est en ouest. Un soulèvement régional plus récent que 2.4 Ma le long de cette ride N65°E est suggérée par plusieurs indices géomorphologiques comme les variations de la géométrie du réseau de drainage, les différences de degré d'incision ou les migrations de rivières de part et d'autre de la ride N65°E. Des données de géophysique et de forages montrent que cette ride se superpose à l'axe d'un anticlinal masqué sous les dépôts alluviaux. Le soulèvement décelé semble donc résulter de la croissance post-2.4 Ma d'un anticlinal. L'azimuth de l'axe de l'anticlinal masqué est oblique aux directions locales des plis et des failles affectant la couverture et, au contraire, est parallèle à la direction des failles du socle anté-triasique connues dans le secteur d'étude ou à proximité, ce qui plaide en faveur d'une réactivation d'une faille du socle paléozoïque de l'avant-pays alpin. La profondeur focale (15 km) du séisme de Besançon du 24 février 2004 plaide également en faveur de cette hypothèse. 1. Introduction Active deformation in the Alps is the consequence of the convergence between the African and European plates which, at the longitude of the Western Alps (8°E), is taking place at a rate of 6,2 ± 0,5 mm.yr-1 in a NNW-SSE direction [DeMets et al., 1990]. Less than half (2~3 mm.yr-1) of this deformation is accommodated across the Western Alps, the remaining being distributed in the Western Mediterranean domain [Calais, 1999; Calais et al., 2000]. A consequence of these low strain rates is that active structures such as faults or folds are difficult to identify. Neither can seismicity be helpful, given the scarcity of instrumentally detected events as well as of historical earthquakes. Conversely, the analysis of the spatial migration of streams through time is a powerful tool to decipher recent tectonic movements [e.g., Jackson et al., 1996; Schumm et al., 2000; Formento-Trigilio et al., 2002]. This geomorphological approach is particularly helpful in weakly active regions such as the foreland of Alpine domain. Particularly in the northern Alpine foreland it has been applied successfully and brought evidence for Post-Pliocene deformation along the outermost part of the Jura fold-and-thrust belt where it intersects with the European Cenozoic Rift system. Thereby most of the previous case studies focused on the area of the southern Upper Rhine Graben [Meyer et al., 1994; Nivière & Winter, 2000; Giamboni et al., 2004a, b and c; Ustaszewski & Schmid, 2007] and the central part of the Rhine Bresse Transfer Zone [Dreyfuss & Glangeaud, 1950; Campy & Contini, 1981; Campy, 1984; Madritsch et al., 2010a and b]. The aim of this contribution is to provide additional geomorphological evidence for Plio-Quaternary tectonic deformation from the transition zone between the Jura fold-and-thrust belt and the northern end of the Bresse Graben. The results are interpreted with respect to the regional structural setting, contribute to a better characterization of the present-day tectonic scenario and have implications on the seismic hazard assessment of the area. 2. Geological outline The study area is located at the junction between the northern end of the Eocene-Oligocene Bresse Graben and the Miocene-Pliocene Jura fold-and-thrust belt (Fig. 1). It consists of a flat-lying depression, the Forêt de Chaux, considered as a branch of the Bresse Graben. In this area, the Jurassic bedrock is unconformably covered by Pliocene alluvial or lacustrine deposits loosely termed hereafter Forêt de Chaux deposits. From top to bottom, they include [Bonvalot, 1974; Chauve et al., 1979]: (1) the upper clay (argiles supérieures) complex, (2) the upper gravel (cailloutis supérieurs) complex, (3) the Etrepigney clay (argiles d'Etrepigney) complex, and (4) the lower gravel (cailloutis inférieurs) complex. The thickness of the Forêt de Chaux deposits is comprised between 35 and 100 m. The thickness of each of the three uppermost complexes is changeable and is generally comprised between 0 and 10 m. The thickness of the lower gravel complex, also changeable, is comprised between 25 and 90 m. The age of deposition of the lower gravel complex is 4.2-2.9 Ma [Chaline, 1984; Puisségur, 1984; Farjanel, 1985; Petit et al., 1996] to 4.2-2.6 Ma [Fejfar et al., 1998]. The age of deposition of the Etrepigney clay complex is estimated at 2.6-2.4 Ma. The lower gravel complex displays strong similarities with the Sundgau deposits found further east, between Montbéliard and Basel (Fig. 1), in the southern end of the Rhine graben. The gravel deposits of both the Forêt de Chaux and the Sundgau areas are of the same petrographical nature and share a common Alpine origin indicated notably by reworked radiolarites. These deposits were transported by an ancestor of the Rhine river which was flowing from the Alps to the Sundgau and the Forêt de Chaux along the N60°E-trending present-day Doubs valley [Liniger, 1967; Bonvalot, 1974; Chauve et al., 1979; Petit et al., 1996; Madritsch et al., 2010a].